1. Field of the Invention
The present invention relates to an image heating apparatus suitable for use as an image heat fixing device mounted to an image forming apparatus such as an electrophotographic copying machine and an electrophotographic printer.
2. Description of the Related Art
As an image heat fixing device (fixing unit) mounted in an electrophotographic image forming apparatus such as a copying machine or a printer, there exists a belt type fixing device. In a belt type fixing device, a nip portion for heating a recording material bearing a toner image such as a recording paper sheet or an OHP sheet while pinching and conveying the same is formed by using an endless belt. As a result, it is possible to secure a large width for the nip portion in the recording material conveyance direction. Thus, if the conveyance speed of the recording material is increased, it is possible to heat the toner image for a sufficient period of time, thus increasing the printing speed. As belt type fixing devices, there have been proposed one in which a belt and a roller are combined and one in which two belts are combined.
Examples of the fixing device in which a belt and a roller are combined are disclosed in Japanese Patent Application Laid-Open No. H10-307496 and Japanese Patent Application Laid-Open No. H06-318001. In those fixing devices, an endless fixing belt is wrapped around two rollers: a fixing roller and a heat roller containing a heat source. Further, between the fixing roller and the heat roller, a pressure roller is brought into contact with the outer peripheral surface of the fixing belt, thereby forming a fixing nip portion.
Examples of the construction in which two belts are combined are disclosed in Japanese Patent Application Laid-Open No. H03-133871 and Japanese Patent Application Laid-Open No. 2004-341346. In those fixing devices, a fixing belt wrapped around a plurality of rollers and a pressure belt wrapped around a plurality of rollers are brought into contact with each other and pressurized, thereby forming a fixing nip portion.